The exhaust gases of internal combustion engines contain pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx) that foul the air. Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants have been set by various governments and must be met by older as well as new vehicles. In order to meet such standards, catalytic converters containing a three way catalyst (TWC) may be located in the exhaust gas line of internal combustion engines. The use of exhaust gas catalysts have contributed to a significant improvement in air quality. The TWC is the most commonly used catalyst and it provides the three functions of oxidation of carbon monoxide (CO), oxidation of unburned hydrocarbons (HC's) and reduction of NOx to N2. TWCs typically utilize one or more platinum group metals (PGM) to simultaneously oxidize CO and HC and reduce NOx compounds. The most common catalytic components of a TWC are platinum (Pt), rhodium (Rh) and palladium (Pd).
TWC catalysts perform best when the engine operates at or close to stoichiometric conditions (air/fuel ratio, λ=1). In actual use, however, engines must operate on either side of λ=1 at various stages during an operating cycle. For example, under rich operating conditions such as during acceleration, the exhaust gas composition is reductive and it is more difficult to carry out oxidation reactions on the catalyst surface. For this reason, TWC's have been developed to incorporate a component which stores oxygen during lean portions of the operating cycle and releases oxygen during rich portions of the operating cycle. This component is ceria-based in most commercial TWC's. Unfortunately, when ceria is doped with precious metal catalysts it tends to lose surface area when exposed to high temperatures, e.g. 800° C. or above, and the overall performance of the catalyst is degraded. TWC's have therefore been developed which use ceria-zirconia mixed oxides as the oxygen storage component, as the mixed oxides are more stable to loss of surface area than ceria alone. TWC catalysts are generally formulated as washcoat compositions containing supports, oxygen storage components and PGMs. Such catalysts are designed to be effective over a specific range of operating conditions which are both lean and rich as compared to stoichiometric conditions.
The platinum group metals in the TWC catalysts (e.g., platinum, palladium, rhodium, rhenium and iridium) are typically disposed on a high surface area, refractory metal oxide support, e.g., a high surface area alumina coating, or on an oxygen storage component. The support is carried on a suitable carrier or substrate such as a monolithic substrate comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. The TWC catalyst substrate may also be a wire mesh, typically a metal wire mesh, which is particularly useful in small engines.
Refractory metal oxides such as alumina, bulk ceria, zirconia, alpha alumina and other materials may be used as supports for the catalytic components of a catalyst article. The alumina support materials, also referred to as “gamma alumina” or “activated alumina,” typically exhibit a BET surface area in excess of 60 square meters per gram (“m2/g”), often up to about 200 m2/g or higher. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. Although many of the other refractory metal oxide supports suffer from the disadvantage of having a considerably lower BET surface area than activated alumina, that disadvantage tends to be offset by a greater durability of the resulting catalyst. Oxygen storage components, such as discussed above, may also be used as supports for the PGM components of the TWC.
In an operating engine, exhaust gas temperatures can reach 1000° C., and such elevated temperatures cause the support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. Alumina supports may be stabilized against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or rare earth metal oxides, such as ceria, lanthana and mixtures of two or more rare earth metal oxides.
Automotive catalyst stability is tested in the laboratory by exposing the catalyst to accelerated aging under laboratory conditions in different atmospheres. These testing protocols mimic operating conditions in the engine, including high temperature and lean/rich perturbations in the exhaust. Such tests typically include high temperature in the presence or absence of water. Two types of accelerated aging protocols are steam/air (oxidative hydrothermal aging, simulating lean operating conditions) or aging under nitrogen, argon or hydrogen (inert aging, simulating rich operating conditions). Although testing under both of these catalyst aging conditions provides better reproduction of catalyst performance in actual use in the engine environment, most attention in the field has been paid to developing catalysts that survive high temperature steam/air aging conditions. Little work has been done to address catalyst stability under high temperature rich aging. Current catalyst technology exhibits significant catalyst deactivation under rich aging conditions, particularly when exposed in sequence to both the steam/air protocol and high temperature rich aging protocols.
In a carbureted motorcycle engine, wide ranges of air to fuel ratios are often encountered as a result of loose control by the carburetor. An emission control catalyst is therefore required to function in this wide range of environments and often loses CO conversion activity under rich aging conditions. Thus, there is a need for a TWC-containing catalyst article with improved CO conversion performance and stability after hydrothermal aging, particularly under rich engine operating conditions. The catalysts of the invention meet this need. It is known that the conversion of CO under rich conditions is accomplished by two reactions: oxidation (CO+½O2═CO2) and water gas shift (WGS) (CO+H2O═CO2+H2). It has now been found that hydrothermal aging processes are more detrimental to the WGS reaction than to the oxidation reaction and that maintaining good PGM dispersion under these conditions is essential for WGS activity. The inventive catalysts described herein exhibit improved PGM dispersion after hydrothermal aging and provide improved catalyst performance.